drm/i915: Translate ENOSPC from shmem_get_page() to ENOMEM

shmemfs first checks if there is enough memory to allocate the page
and reports ENOSPC should there be insufficient, along with
the usual ENOMEM for a genuine allocation failure.

We use ENOSPC in our driver to mean that we have run out of aperture
space and so want to translate the error from shmemfs back to
our usual understanding of ENOMEM. None of the the other GEM users
appear to distinguish between ENOMEM and ENOSPC in their error handling,
hence it is easiest to do the fixup in i915.ko

Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>